Gaseous discharge tube



Oct. 14, 1958 E LYNCH 2,856,551

GASEOUS DISCHARGE TUBE Filed Nov. 1, 1956 I2 I I INVENTOR.

ATTORNEY R bert E. Lynch GASEOUS DISCHARGE TUBE Appiication November 1,1956, SerialNo. 619,846

1 Claim. or. sis-54y The invention pertains to gaseous discharge tubesand particularly to the gaseous filling of such tubes.

Two general types of gaseous discharge tubes are known in the art; theso-called hot-cathodegas tube and the glow discharge tube. They aresimilar in that almost complete ionization of a gaseous filling isrequired to attain the desired mode of operation. The ionization currentin both types is substantially independent of the potential appliedacross the electrodes therein, so both may be considered to be constantcurrent devices. The dissimilarities between the two types center mainlyin the mechanism by which the required ionization is initiated.

In the hot-cathode gas tube, a thermionic cathode furnishes a source ofelectrons toxinitiate ionization of the gas. When the potentialdifference betweenthe cathode and the anode exceeds a certain criticalvalue (about to volts in the ordinary case), sufficient kinetic energyis imparted to the emitted electrons to cause ionization of the gas bycollision. Once the process of ionization has begun, successivecollisions between the individual molecules of the gas multiply untilsaturation is substantially attained. The ionization current thenreaches a stabilized and constant value, although a small increase inthis current may be noted if the potential difference is increasedconsiderably.

The glow discharge tube, however, has a cold or non thermionic cathode.Ionization is initiated by applying a potential difference (at least 100volts in the ordinary case) between the anode and the cathode. Anyionized molecules present in the gas filling the tube are, therefore,accelerated toward the appropriate electrode. However, the mean freepath of such ionized molecules is'so short in comparison to the distancebetween the electrodes that collisions between ionized and non-ionizedmolecules are certain to occur. The process is cumulative until asubstantially constant ionization current is passed by the tube in theform of a glow type of discharge.

The exact potential required to create a glow discharge depends on manyfactors. Some, such as the geometry of the electrodes and type andpressure of the gas used, may determine constant parameters establishingthe operation of the tube. Other factors, such as the degree of residualionization of the gas when operation is initiated, must be treated asvariables. While operationin accordance with desired constantparameters" may be obtained by proper design and by care in manufacture,the degree of residual ionization, depending as it does on theconditions under which the tube is used, cannot so be controlled in thecommon glow discharge device.

Independence of the degree of residual ionization of the gas in a glowdischarge tube from environmental conditions may be attained byincorporating an ionizing source within the tube itself. More than oneway of providing an ionizing source within a glow discharge tube isknown. Thus, in some glow discharge tubes, selfluminous materials havebeen used to provide sufficient luminous flux to maintain a certainminimum degree of ionization of the gas and in other such tubes solidnited States Patent 0 radioactive materials have been disposed onvarious electrodes to accomplish the same purpose by radioactiveemission. Of these two ways, the latter has proved the more efficientand offers the better promise of attaining the desired end.

Great difficulties are encountered in the use of a solid radioactivematerial for stabilizing the striking voltage of a glow discharge tube.Briefly, the difficulties encountered are: (1) The radioactive materialmay contaminate the electrodes in the tube to cause malfunctioning; (2)harmful radiations therefrom expose using or handling personnel todanger; and (3) safe disposal of a tube containing a solid radioactivematerial is not easy when the tube has reached the end of its life.Unfortunately, all known solid radioactive materials exhibit at leastone of the enumerated deficiencies to a serious degree.

In addition to freedom from the aforementioned difficulties, anyradioactive material used as an ionizing source in a tube should have ahalflife of at least three years. In view of the time lag normallyencountered between manufacturing and using a tube and the lifeexpectancy of any tube in operation, the benefits and ad vantages of aradioactive ionizing source will not be obtained, in many cases, if thehalf-life of the radioactive material is less. While there are manyradioactive materials having a half-life greater than the desiredminimum, almost all such materials are either too expensive or toodangerous to use in glow discharge tubes. Further, the safety with whichthe radioactive material may be handled is of primary importance duringmanufacture of tubes using radioactive material. Obviously, althoughefiiciency demands that a relatively large supply of radioactivematerial be maintained at the manufacturing site, the hazards to healthinherent in concentration of radioactive materials make it highlydesirable that only small quantities of such materials be kept in anyone place. As a result, a sufiiciently large quantity of almost allradioactive materials to attain a high degree of efliciency inmanufacturing may not be safely assembled. Even though establishedsafety practices allow 300 mr absorption of radiation per week as themaximum safe dose rate, it is recognized that such a limit should beconsidered for guidance only and not as a limit to justify unnecessaryexposure. If exposure can be reduced by effort in selecting safermaterials and developing better techniques, the expense of effectingsuch reductions is well worth while. This is especially true in view ofthe psychological effect of improving working conditions for persons whoare not too familiar with the mechanism by which radiations affect thehuman body.

Another problem that affects the use of radioactive materials in tubesis that of contamination. If the material used can diffuse into theelectrodes or the envelope, potential hazard is created. Once any partbecomes irradiated, it is as dangerous as the irradiating source andmust be treated accordingly. Therefore, it is highly important thatcontamination be avoided.

An object of the invention is to provide a safe, longlived andinexpensive ionizing source for glow discharge tubes;

Another object is to provide a radioactive ionizing source that does notaffect electrical characteristics of glow discharge tubes other than toimprove the stability of the striking voltage of the glow dischargetherein under all operating conditions;

A further object of the invention is to provide a radioactive ionizingsource that may be safely and easily used in the construction of glowdischarge tubes;

A further object of the invention is to increase the ease with whichglow discharge tubes containing radioactive materials may bemanufactured and distributed;

A still further object of the invention is to minimize the possibilityof contaminating the electrodes in glow discharge tubes with radioactivematerial used therein.

According to the invention, the desired results may be obtained by usinga radioactive gas in filling a glow discharge tube. The gas should havea long life and emit only alpha or beta emanations whereby a desiredminimum degree of ionization of the gas may be maintained in a glowdischarge tube during the entire life expectancy of the tube. Theradioactive gas may be mixed with any of the gases normally used to fillglow discharge tubes as a trace element, being added automatically fromimpermeable containers adapted to dispense the correct amount of gasinto each tube envelope. The amount of radioactive gas in each tube iskept so low that, even if a large number of envelopes were to be brokensimultaneously, there will be no ill effects to human beings. Further,in the event of accident, the radioactive material would be quicklydispersed into the atmosphere. However, the radioactivity of the gas iskept sufficiently high so that its alpha or beta emanations continuouslyionize the gas in the tube to maintain a stable striking voltage. At thesame time, after the life of the tube has expired, disposal of theradioactive gas is easily accomplished by breaking the envelope of thetube to allow the radioactive gas to be dissipated safely in theatmosphere.

In order that all of the practical advantages may readily be attained,an express embodiment, as example only, of the invention is describedhereinafter with reference to the accompanying drawing in which the solefigure illustrates a glow discharge tube incorporating the invention.

There is shown in the drawing, a tube consisting of a gas tight envelope11, preferably of glass, having a plurality of pins 12 and leads 13sealed in the header portion of the envelope. Individual ones of theleads 13 are connected to a cathode electrode 14 and an anode electrode15. A starting electrode 16 is integrally aflixed to the cathodeelectrode 14 as shown. While the particular shape illustrated is notessential to the present invention, it may be used to advantage asdescribed in the co-pending U. S. patent application, Serial No.463,846, of R. R. Law, entitled Voltage Regulator, filed October 22,1954, now U. S. Patent No. 2,774,906 and assigned to the same assigneeas the present application. The cathode electrode 14 and anode electrodeare held in position with respect to each other and with the envelope 11by means of electrically insulating spacing members 17 commonly made ofmica. A gas 18 that may, for example, be a mixture of helium 99%; argon0.99%; and krypton 85, 0.01%, by volume, at a pressure of 100 mm. Hg.,fills the empty space within the envelope 11. The individual gases ofthe mixture may be changed in proportion or in kind, except for thekrypton 85, or the pressure of the gas may be varied in order to changethe operating characteristics of the tube. It is desirable, however,that the krypton 85 be the longer-lived isotope of krypton which is freeof gamma rays and that its concentration be kept as low as possibleconsistent with obtaining the desired results in operation as will beexplained in more "detail hereinafter. In this connection, it has beenfound that concentrations as low as 0.01 to 0.02 mc./cc. aresatisfactory. Further, when precautions need be taken only against alphaand beta emanations and not against gamma rays, it is evident thatmanufacturing eificiency will increase because of the increased sense ofsecurity given workers.

The introduction of krypton 85 into the envelope 11 presents noparticular problem to those skilled in the art. Thus, a mixture of gascontaining the proper amount of krypton 85 may be stored in a protectedtank placed adjacent the machine used to exhaust and seal the envelope11 during manufacturing. By connecting appropriately shaped piping andusing the proper kind of valve (both of which are well known), the exactamount of gas required to fill the envelope of each tube may be meteredand injected into the envelope immediately after the envelope has beenevacuated and the tube elements outgassed. Outgassing may be completedeasily by a getter assembly 19 of any known type. This operation may beaccomplished by automatic machinery so there is no danger to personneleven if the machinery breaks down.

In operation, the tube performs in exactly the same way as theconventional glow discharge tube, except for a greatly improvedstability of the striking voltage characteristic brought about by theinvention. That is, the potential difference between the startingelectrode attached to the cathode electrode and the anode electroderequired to initiate a glow discharge between the cathode and anodeelectrodes does not vary appreciably as operating conditions change.This stability is especially evident when the so-called dark startingvoltage is considered. If a tube without an internal source ofionization is kept shielded from light for any appreciable time, it willbe found that the striking voltage will vary erratically. However, atube embodying the radioactive gas of the invention will be free of suchfailing. In addition, the materials used in glow discharge tubes adsorbonly negligible amounts of krypton 85, so the possibility ofcontamination may be disregarded.

When a tube constructed according to the invention becomes inoperativefor any reason, it is a simple matter to render it harmless. By merelybreaking the envelope, the gas filling therein is freed to mix with thesurrounding atmosphere. The very small amount of krypton 85 almostinstantly is so diluted that it is no longer even slightly dangerous.

The invention claimed is:

A glow discharge tube containing a gaseous mixture having a pressure inthe range of to 100 mm. of mercury, said gaseous mixture comprising,krypton in an amount of 1 part in 10,000 by volume thereof to provide aradioactive gas emanating only alpha and beta particles and auniformradioactive density within said glow discharge tube of from .01 to .02microcurie per centimeter, whereby the striking potential of said glowdischarge tube may be effectively stabilized and the remainder of saidgaseous mixture consisting of an inert gas selected from the group ofhelium, argon and neon.

References Cited in the file of this patent UNITED STATES PATENTS2,354,786 Wall Aug. 1, 1944 2,375,130 Perrin et al. May 1, 19452,576,100 Brown Nov. 27, 1951 2,616,986 Coleman Nov. 4, 1952 2,678,397Herzog May 11, 1954 Flu-m

